1. Field of the Invention
This invention relates to telephoto objectives, and more particularly to compact high-performance photographic objectives of long focal length.
2. Description of the Prior Art
Aberrations affecting the imaging performance of telephoto objectives are the well known chromatic aberrations and curvature of field. Of these the longitudinal chromatic aberration though increasing proportionally with increase in the focal length can be corrected to an extremely small quantity by using fluorite, or special low-dispersion glass in the positive lens of the front group of the lens system. Also the field curvature can be well corrected to some degree by the choice of reasonable glasses (those of low refractive index in the positive lens, and of high refractive index in the negative lens), provided the size of the lens system is not reduced much. But, when the lens system is made so compact that the telephoto ratio lies between 0.6 and 0.7 as in specific embodiments of the present invention, it has to allow for deterioration of other aberrations. In particular, spherical aberration and coma and further chromatic aberrations, despite good correction of field curvature.
In the art of telephoto objectives, it has been known to construct the front group of four lenses for the purpose of achieving an increase in the imaging performance, as, for example, U.S. Pat. No. 4,176,913 and Japanese Pat. 46-14423.